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Oil Well cements have been developed for use in well grouting in oil and gas wells. They can also be used for sealing water
wells, waste disposal pits and geo-thermal wells.
Primary purpose of Oil Well cement is to seal off the annular space between steel casing and
rock strata and also to seal off any other fissures or cavities in the rock. This
prevents escape of oil/gas.
Generally the basic requirements are:-
1. They must
be slow-setting (slurry should remain sufficiently mobile for several
hours) even under the high temperatures obtained in oil wells; and
2. They
must be resistant to high temperatures and pressures.
These cements are designed to set at
high temperatures (upto 1750C at considerable depth) and
pressures (upto 1300 kg/cm2) in well grouting.
In order to meet these requirements, oil well cements are made
with special composition or additives enabling them to have low
permeability, good bond between rock and casing, to withstand and set under
high temperature and pressure and to protect the casing against corrosion (
from sulphur gases or waters containing dissolved salts) and collapse.
Oil
well cement can be produced either by adjusting compound composition of cement
or by adding retarders (like starches/cellulose products/acids). Workability
agents may also be added.
The American Petroleum Institute’s Specifications for Materials and Testing for
Well Cements (API Specification 10A) includes requirements for eight
classes of oil-well cements (classes A through H) and three grades
viz:
O - Ordinary,
MSR - Moderate sulphate resistant, and
HSR - High sulphate resistant.
Each
class is applicable for use at a certain range of well depths, temperatures,
pressures, and sulphate environments. The petroleum industry also uses conventional
types of Portland cement with suitable cement-modifiers. Expansive cements
have also performed adequately as well cements.
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